Conjugation or mating in paramecia is characterized by a highly specific cell-cell interaction which appears to be mediated through receptor sites found on the cilia and surface of the pellicle (cell membrane). When cells of the opposite or complementary mating type are mixed they adhere to one another on contact forming clumps. Within minutes, the initial agglutination reaction is followed by a pairing of the complimentary cells. The process of specific cell agglutination appears to be the result of a membrane associated protein or proteins called mating type substances of M-agglutimins. The project outlined in this report is designed to isolate and chemically characterize this substance(s) and define the molecular basis for the specific interaction.